Secondary batteries have been widely spread not only as power sources of portable devices such as mobile phones, digital cameras and laptop computers but also as vehicle or household power sources. In particular, a lightweight lithium ion secondary battery with a high-energy density is an energy storage device that is indispensable for our life.
The secondary battery can be roughly categorized as a wound type or a laminated type. A battery element of the wound-type secondary battery has a structure in which a long positive electrode sheet and a long negative electrode sheet are wound multiple times in a state of being overlapped with separators respectively interposed therebetween. A battery element of the laminated-type secondary battery has a structure in which positive electrode sheets and negative electrode sheets are laminated alternately and repeatedly with separators respectively interposed therebetween. The positive electrode sheet and the negative electrode sheet each include an application portion where an active material layer (including a case of a mixture agent including a binding agent, a conductive material and the like as well as an active material) is formed on a current collector, and a non-application portion where the active material layer is not formed for the connection with an electrode terminal.
In each of the wound-type secondary battery and the laminated-type secondary battery, together with an electrolyte, the battery element is accommodated and sealed in an outer container (outer case). Then, one end of a positive electrode terminal is electrically connected with the non-application portion of the positive electrode sheet while the other end is led out of the outer container, and one end of a negative electrode terminal is electrically connected with the non-application portion of the negative electrode sheet while the other end is led out of the outer container. With yearly improvements in battery technology, the trend is for annual increase in the capacity of the secondary batteries. What this means is that, should a short circuit occur, the amount of generated heat will increase which, in turn, increases safety risks. Therefore, measures to improve battery safety become more and more important.
As an example of the safety measure, as shown in Patent Document 1, there is a technology in which an insulating member is provided at a border portion between the application portion and the non-application portion to prevent a short circuit from occurring between the positive electrode and the negative electrode. The technology disclosed in Patent Document 1 disposes, on a positive electrode current collector of the positive electrode, an insulating member that covers a border portion between an application portion where a positive electrode active material layer is formed and a non-application portion where the positive electrode active material layer is not formed. In the laminated-type secondary battery, the insulating members are repeatedly laminated at the same position in the planar view. Therefore, at the position where the insulating member is disposed, the thickness of a part of the battery element is large, and the energy density per unit volume decreases.
Further, in the secondary battery, to stabilize the electric characteristic and reliability, it is preferable to fix the battery element by a tape or the like and apply uniform pressure to the battery element. However, when the insulating member in Patent Document 1 is used in the laminated-type secondary battery, it is not possible to apply uniform pressure to the battery element due to the thickness difference between the portion where the insulating member is present and the portion where the insulating member is not present, and there is concern over causing a decrease in battery quality such as the variability in the electric characteristics and a decrease in cycle characteristics.
Patent Document 2 proposes a configuration of thinning the portion where the insulating member on the electrode (positive electrode) is disposed and thereby preventing a partial increase in the thickness of the battery element. Patent Document 2 suggests a method of continuously forming the active material on the current collector, and a method of intermittently applying the active material.
Patent Document 3 describes an intermittent supply valve that is used for the intermittent application of the active material.